Long-Term Debt (Details)	3 Months Ended	6 Months Ended		12 Months Ended
	Mar. 31, 2017 USD ($) loan	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Mar. 31, 2018
Debt Instrument
Long-term debt		$ 14,192,000,000		$ 12,162,000,000
Proceeds from issuance of long-term debt		4,050,000,000	$ 0
Parent Company [Member]
Debt Instrument
Long-term debt		6,298,000,000		6,997,000,000
Parent Company [Member] | Senior debt obligations | Floating rate senior notes due February 2018. These notes, which bore interest at 0.57% above 3-month LIBOR, had a rate of 1.97% at December 31, 2017
Debt Instrument
Long-term debt		$ 0		$ 250,000,000
Long-term debt, weighted average interest rate (as a percent)		0.00%		1.97%
Parent Company [Member] | Senior debt obligations | Floating rate senior notes due February 2018. These notes, which bore interest at 0.57% above 3-month LIBOR, had a rate of 1.97% at December 31, 2017 | London Interbank Offered Rate LIBOR
Debt Instrument
Interest rate above variable interest rate (as a percent)		0.00%		0.57%
Parent Company [Member] | Senior debt obligations | Fixed rate 1.625% notes due February 2018
Debt Instrument
Long-term debt		$ 0		$ 450,000,000
Debt instrument, fixed interest rate (as a percent)		1.63%
Parent Company [Member] | Senior debt obligations | Fixed rate 2.25% notes due February 2020
Debt Instrument
Long-term debt		$ 998,000,000		998,000,000
Debt instrument, fixed interest rate (as a percent)		2.25%
Parent Company [Member] | Senior debt obligations | Fixed rate 3.50% notes due June 2022
Debt Instrument
Long-term debt		$ 398,000,000		398,000,000
Debt instrument, fixed interest rate (as a percent)		3.50%
Parent Company [Member] | Senior debt obligations | Fixed rate 3.00% notes due February 2025
Debt Instrument
Long-term debt		$ 496,000,000		496,000,000
Debt instrument, fixed interest rate (as a percent)		3.00%
Parent Company [Member] | Senior debt obligations | Floating rate debt due March 2020. This note, which bears interest at 0.86% above 3-month LIBOR, had a rate of 3.21% at June 30, 2018 and 2.45% at December 31, 2017
Debt Instrument
Long-term debt		$ 545,000,000		$ 545,000,000
Long-term debt, weighted average interest rate (as a percent)		3.21%		2.45%
Parent Company [Member] | Senior debt obligations | Floating rate debt due March 2020. This note, which bears interest at 0.86% above 3-month LIBOR, had a rate of 3.21% at June 30, 2018 and 2.45% at December 31, 2017 | London Interbank Offered Rate LIBOR
Debt Instrument
Interest rate above variable interest rate (as a percent)		0.86%		0.86%
Parent Company [Member] | Senior debt obligations | Floating rate debt due September 2020. This note, which bears interest at 0.85% above 3-month LIBOR, had a rate of 3.20% at June 30, 2018 and 2.54% at December 31, 2017
Debt Instrument
Long-term debt		$ 3,500,000,000		$ 3,500,000,000
Long-term debt, weighted average interest rate (as a percent)		3.20%		2.54%
Proceeds from issuance of long-term debt	$ 3,500,000,000
Prepayment of long term debt		$ 500,000
Parent Company [Member] | Senior debt obligations | Floating rate debt due September 2020. This note, which bears interest at 0.85% above 3-month LIBOR, had a rate of 3.20% at June 30, 2018 and 2.54% at December 31, 2017 | London Interbank Offered Rate LIBOR
Debt Instrument
Interest rate above variable interest rate (as a percent)		0.85%
Parent Company [Member] | Senior debt obligations | Floating rate debt due December 2023. This note, which bears interest at 0.76% above 3-month EURIBOR, had a rate of 0.76% at June 30, 2018 and 0.76% at December 31, 2017
Debt Instrument
Long-term debt		$ 24,000,000		$ 24,000,000
Long-term debt, weighted average interest rate (as a percent)		0.76%		0.76%
Parent Company [Member] | Senior debt obligations | Floating rate debt due December 2023. This note, which bears interest at 0.76% above 3-month EURIBOR, had a rate of 0.76% at June 30, 2018 and 0.76% at December 31, 2017 | London Interbank Offered Rate LIBOR
Debt Instrument
Interest rate above variable interest rate (as a percent)		0.76%
Parent Company [Member] | Subordinated Debt | Floating rate subordinated debt due December 2023. This note, which bears interest at 1.38% above 3-month LIBOR, had a rate of 3.73% at June 30, 2018 and 3.07% at December 31, 2017
Debt Instrument
Long-term debt		$ 300,000,000		$ 300,000,000
Long-term debt, weighted average interest rate (as a percent)		3.73%		3.07%
Parent Company [Member] | Subordinated Debt | Floating rate subordinated debt due December 2023. This note, which bears interest at 1.38% above 3-month LIBOR, had a rate of 3.73% at June 30, 2018 and 3.07% at December 31, 2017 | London Interbank Offered Rate LIBOR
Debt Instrument
Interest rate above variable interest rate (as a percent)		1.38%		1.38%
Parent Company [Member] | Junior subordinated debt payable to trusts | Floating rate note due September 2036. This note had an interest rate of 4.04% at June 30, 2018 and 3.29% at December 31, 2017
Debt Instrument
Long-term debt		$ 37,000,000		$ 36,000,000
Long-term debt, weighted average interest rate (as a percent)		4.04%		3.29%
MUFG Union Bank N.A
Debt Instrument
Long-term debt		$ 6,734,000,000		$ 3,810,000,000
MUFG Union Bank N.A | Senior debt obligations | Floating rate debt due September 2020. This note, which bears interest at 0.85% above 3-month LIBOR, had a rate of 3.20% at June 30, 2018 and 2.54% at December 31, 2017 | MUFG Americas Holding Corporation
Debt Instrument
Number of loans from BTMU prepaid | loan	3
Repayments of debt	$ 3,500,000,000
MUFG Union Bank N.A | Senior debt obligations | Fixed rate 2.63% notes due September 2018
Debt Instrument
Long-term debt		$ 1,000,000,000		$ 1,000,000,000
Debt instrument, fixed interest rate (as a percent)		2.63%		2.63%
MUFG Union Bank N.A | Senior debt obligations | Fixed rate FHLB of San Francisco advances due between July 2018 and June 2020. These notes bear a combined weighted-average rate of 2.06% at June 30, 2018 and 1.51% at December 31, 2017
Debt Instrument
Long-term debt		$ 5,200,000,000		$ 1,500,000,000
Debt instrument, fixed interest rate (as a percent)		1.85%		1.51%
MUFG Union Bank N.A | Senior debt obligations | Fixed rate 2.25% notes due May 2019
Debt Instrument
Long-term debt		$ 495,000,000		$ 497,000,000
Debt instrument, fixed interest rate (as a percent)		2.25%		2.25%
MUFG Union Bank N.A | Subordinated Debt | Floating rate subordinated debt due June 2023. This note, which bore interest at 1.20% above 3-month LIBOR, had a rate 2.89% at December 31, 2017
Debt Instrument
Long-term debt		$ 0		$ 750,000,000
Long-term debt, weighted average interest rate (as a percent)				2.89%	3.49%
MUFG Union Bank N.A | Subordinated Debt | Floating rate subordinated debt due June 2023. This note, which bore interest at 1.20% above 3-month LIBOR, had a rate 2.89% at December 31, 2017 | London Interbank Offered Rate LIBOR
Debt Instrument
Interest rate above variable interest rate (as a percent)		1.20%		1.20%
MUFG Union Bank N.A | Other
Debt Instrument
Long-term debt		$ 39,000,000		$ 63,000,000
Other Subsidiaries, Excluding MUFG Bank, NA
Debt Instrument
Long-term debt		1,160,000,000		1,355,000,000
Other Subsidiaries, Excluding MUFG Bank, NA | Senior debt obligations | Various floating rate borrowings due between December 2020 and May 2021. These notes, which bear interest above 3-month LIBOR had a weighted-average interest rate of 2.34% at June 30, 2018 and 1.78% at December 31, 2017
Debt Instrument
Long-term debt		250,000,000		$ 291,000,000
Long-term debt, weighted average interest rate (as a percent)				1.78%	2.38%
Other Subsidiaries, Excluding MUFG Bank, NA | Senior debt obligations | Various fixed rate borrowings due between February 2019 and May 2024 with a weighted-average interest rate of 1.83% (between .14% and 2.44%) at June 30, 2018 and 2.12% (between 1.37% and 2.65%) at December 31, 2017
Debt Instrument
Long-term debt		286,000,000		$ 339,000,000
Long-term debt, weighted average interest rate (as a percent)				2.12%	2.13%
Other Subsidiaries, Excluding MUFG Bank, NA | Senior debt obligations | Various fixed rate borrowings due between February 2019 and May 2024 with a weighted-average interest rate of 1.83% (between .14% and 2.44%) at June 30, 2018 and 2.12% (between 1.37% and 2.65%) at December 31, 2017 | Minimum
Debt Instrument
Long-term debt, weighted average interest rate (as a percent)				1.71%	1.37%
Other Subsidiaries, Excluding MUFG Bank, NA | Senior debt obligations | Various fixed rate borrowings due between February 2019 and May 2024 with a weighted-average interest rate of 1.83% (between .14% and 2.44%) at June 30, 2018 and 2.12% (between 1.37% and 2.65%) at December 31, 2017 | Maximum
Debt Instrument
Long-term debt, weighted average interest rate (as a percent)				2.44%	2.65%
Other Subsidiaries, Excluding MUFG Bank, NA | Senior debt obligations | Various floating rate borrowings due between September 2018 and March 2019. These notes, which bear interest above 6-month LIBOR had a weighted-average interest rate of 3.93% (between 3.83% and 3.98%) at June 30, 2018 and 2.95% (between 2.88% and 3.04%) at December 31, 2017
Debt Instrument
Long-term debt, weighted average interest rate (as a percent)				2.95%	3.93%
Other Subsidiaries, Excluding MUFG Bank, NA | Senior debt obligations | Various floating rate borrowings due between September 2018 and March 2019. These notes, which bear interest above 6-month LIBOR had a weighted-average interest rate of 3.93% (between 3.83% and 3.98%) at June 30, 2018 and 2.95% (between 2.88% and 3.04%) at December 31, 2017 | Minimum
Debt Instrument
Long-term debt, weighted average interest rate (as a percent)				2.88%	3.83%
Other Subsidiaries, Excluding MUFG Bank, NA | Senior debt obligations | Various floating rate borrowings due between September 2018 and March 2019. These notes, which bear interest above 6-month LIBOR had a weighted-average interest rate of 3.93% (between 3.83% and 3.98%) at June 30, 2018 and 2.95% (between 2.88% and 3.04%) at December 31, 2017 | Maximum
Debt Instrument
Long-term debt, weighted average interest rate (as a percent)				3.04%	3.98%
Other Subsidiaries, Excluding MUFG Bank, NA | Subordinated Debt | Various floating rate borrowings due between September 2018 and March 2019. These notes, which bear interest above 6-month LIBOR had a weighted-average interest rate of 3.93% (between 3.83% and 3.98%) at June 30, 2018 and 2.95% (between 2.88% and 3.04%) at December 31, 2017
Debt Instrument
Long-term debt		110,000,000		$ 185,000,000
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Various floating rate non-recourse borrowings due between November 2018 and December 2021. These notes, which bear interest above 1- or 3-month LIBOR had a weighted-average interest rate of 3.70% (between 2.34% and 4.48%) at June 30, 2018 and 3.07% (between 1.49% and 5.58%) at December 31, 2017
Debt Instrument
Long-term debt		90,000,000		$ 79,000,000
Long-term debt, weighted average interest rate (as a percent)				3.07%	3.39%
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Various floating rate non-recourse borrowings due between November 2018 and December 2021. These notes, which bear interest above 1- or 3-month LIBOR had a weighted-average interest rate of 3.70% (between 2.34% and 4.48%) at June 30, 2018 and 3.07% (between 1.49% and 5.58%) at December 31, 2017 | Minimum
Debt Instrument
Long-term debt, weighted average interest rate (as a percent)				1.49%	2.11%
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Various floating rate non-recourse borrowings due between November 2018 and December 2021. These notes, which bear interest above 1- or 3-month LIBOR had a weighted-average interest rate of 3.70% (between 2.34% and 4.48%) at June 30, 2018 and 3.07% (between 1.49% and 5.58%) at December 31, 2017 | Maximum
Debt Instrument
Long-term debt, weighted average interest rate (as a percent)				5.58%	3.85%
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Fixed rate non-recourse borrowings due between January 2019 and July 2023 which had an interest rate of 3.07% at June 30, 2018 and 3.27% at December 31, 2017
Debt Instrument
Long-term debt		204,000,000		$ 240,000,000
Long-term debt, weighted average interest rate (as a percent)				3.27%	3.24%
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Various floating rate non-recourse borrowings due between December 2018 and May 2019. These notes, which bear interest above 1- or 3-month LIBOR had a weighted-average interest rate of 3.58% (between 3.33% and 4.2%) at June 30, 2018 and 2.88% (between 2.50% and 3.54%) at December 31, 2017
Debt Instrument
Long-term debt		186,000,000		$ 185,000,000
Long-term debt, weighted average interest rate (as a percent)				2.88%	3.58%
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Various floating rate non-recourse borrowings due between December 2018 and May 2019. These notes, which bear interest above 1- or 3-month LIBOR had a weighted-average interest rate of 3.58% (between 3.33% and 4.2%) at June 30, 2018 and 2.88% (between 2.50% and 3.54%) at December 31, 2017 | Minimum
Debt Instrument
Long-term debt, weighted average interest rate (as a percent)				2.50%	3.33%
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Various floating rate non-recourse borrowings due between December 2018 and May 2019. These notes, which bear interest above 1- or 3-month LIBOR had a weighted-average interest rate of 3.58% (between 3.33% and 4.2%) at June 30, 2018 and 2.88% (between 2.50% and 3.54%) at December 31, 2017 | Maximum
Debt Instrument
Long-term debt, weighted average interest rate (as a percent)				3.54%	4.20%
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Fixed rate non-recourse borrowings due December 2026 which had an interest rate of 5.34% at June 30, 2018 and December 31, 2017
Debt Instrument
Long-term debt		$ 34,000,000		$ 36,000,000
Debt instrument, fixed interest rate (as a percent)				5.34%	5.34%